Add function to Issuer.sol to Add Synths in batches

[jacko125]

Currently when Issuer.sol is upgraded and released, the Synths have to be re-added via 40+ individual transactions using addSynth per synth.

function addSynth(ISynth synth) external onlyOwner {
        bytes32 currencyKey = synth.currencyKey();

        require(synths[currencyKey] == ISynth(0), "Synth already exists");
        require(synthsByAddress[address(synth)] == bytes32(0), "Synth address already exists");

        availableSynths.push(synth);
        synths[currencyKey] = synth;
        synthsByAddress[address(synth)] = currencyKey;

        emit SynthAdded(currencyKey, address(synth));
    }

We can add a addSynths(Isynth[] synths) function to add the synths in batches on deployment cutting down the cost of deploying a new Issuer contract.